1. Field of the Invention
The present invention relates to a fuel cell power generating system and a method of controlling the same. Specifically, this invention relates to a fuel cell power generating system having two fuel cell stacks of different types and a method of controlling the same.
2. Description of the Related Art
FIG. 1 shows a configuration of a conventional fuel cell power generating system. The conventional system is a polymer electrolyte fuel cell system which uses natural gas as a fuel. The conventional system primarily comprises a desulfurizer 2, a reformer 3, a reformer burner 53, a CO shift converter 4, a CO selective oxidizer 5, a condenser 39, a polymer electrolyte fuel cell stack 9, a power adjusting device 20, a carbureter 14, a carbureter burner 35, a water tank 90, flow control valves (10, 11, 12, . . . ), a feed water pump 42, an air supply blower 13 and pipes or the like that connect these components to each other.
Reference numerals in FIG. 1 will be now described. Reference numeral 1 denotes a natural gas serving as a fuel, reference numeral 2 denotes a desulfurizer that removes sulfur from the natural gas 1, and reference numeral 3 denotes a reformer that causes a steam reforming reaction of the fuel. Reference numeral 4 denotes a CO shift converter that converts carbon monoxide (CO) resulting from the steam reforming reaction into carbon dioxide by water shift reaction, thereby providing hydrogen. Reference numeral 5 denotes a CO selective oxidizer that oxidizes carbon monoxide remaining after the water shift reaction to form carbon dioxide.
Reference numeral 9 denotes a polymer electrolyte fuel cell stack, and reference numerals 6, 7 and 8 denote an anode, a solid polymer electrolyte and a cathode, respectively, of the polymer electrolyte fuel cell stack 9. Reference numerals 10, 11 and 12 denote flow control valves that control the flow rate of air 18 from an air supply blower 13. Reference numeral 14 denotes a carbureter that produces steam used for the steam reforming reaction. Reference numeral 15 denotes a pump for the carbureter 14, and reference numeral 16 denotes steam produced by the carbureter 14.
Reference numeral 17 denotes a cathode exhaust gas from the polymer electrolyte fuel cell stack 9, reference numeral 18 denotes air from the air supply blower 13 and reference numeral 19 denotes an anode exhaust gas from the polymer electrolyte fuel cell stack 9. Reference numeral 20 denotes a power adjusting device, reference numeral 21 denotes a load, reference numeral 22 denotes a DC output power from the polymer electrolyte fuel cell stack 9, reference numeral 23 denotes an AC output power at a sending end, and reference numeral 24 denotes a combustion exhaust gas from a reformer burner 53. Reference numeral 25 denotes a reformed gas having a CO concentration reduced to about 10 ppm, which is an exhaust gas from the CO selective oxidizer 5. Reference numeral 26 denotes a reformed gas having a CO concentration reduced to 1% or lower, which is an exhaust gas from the CO shift converter 4. Reference numeral 27 denotes a reformed gas rich in hydrogen at an outlet of the reformer 3, which is an exhaust gas from the reformer 3.
Reference numeral 29 denotes a desulfurized natural gas, which is an exhaust gas from the desulfurizer 2, and reference numeral 28 denotes a mixture gas of steam and the desulfurized natural gas. Reference numeral 30 denotes a flow control valve that controls the flow rate of the air 18 from the air supply blower 13. Reference numeral 36 denotes a combustion exhaust gas from the carbureter burner 35, and reference numeral 37 denotes a flow control valve that controls the flow rate of the natural gas 45. Reference numeral 31 denotes air for a carbureter burner 35, reference numeral 32 denotes air for the polymer electrolyte fuel cell stack 9, reference numeral 33 denotes air for the CO selective oxidizer 5, and reference numeral 34 denotes air for the reformer burner 53.
Reference numeral 39 denotes a condenser that condenses moisture in the reformed gas 25, which is the exhaust gas from the CO selective oxidizer 5. Reference numeral 38 denotes a reformed gas resulting after the condenser 39 condenses unreacted steam. Reference numeral 40 denotes water produced by the cell reaction in the polymer electrolyte fuel cell stack 9, and reference numeral 41 denotes a condensate produced by the condenser 39. Reference numeral 42 denotes a feed water pump, reference numeral 43 denotes feed water, and reference numeral 44 denotes water to be supplied to the carbureter 14.
Reference numeral 45 denotes a natural gas to be supplied to the desulfurizer 2, reference numeral 46 denotes a natural gas for the carbureter burner 35, reference numerals 47 and 48 denote flow control valves that control the flow rates of the natural gases 45 and 49, respectively, and reference numeral 49 denotes a natural gas for the reformer burner 53. Reference numeral 50 denotes are cycled reformed gas to the desulfurizer 2. Reference numeral 51 denotes a flow control valve that controls the flow rate of the recycled reformed gas 50. Reference numeral 52 denotes a reformed gas for the CO selective oxidizer 5. Reference numeral 53 denotes the reformer burner as described above, reference numeral 90 denotes a water tank, reference numeral 91 denotes an exhaust gas from the carbureter 14, and reference numeral 96 denotes a flow control valve that controls the flow rate of the steam 16 from the carbureter 14.
The phrase “rich in hydrogen” above means that there is contained enough hydrogen to contribute to power generation through the cell reaction.
For the sake of convenience, FIG. 1 shows the polymer electrolyte fuel cell stack 9 constituted by a unit cell consisting of a set of the anode 6, the solid polymer electrolyte 7 and the cathode 8. In practical, however, the polymer electrolyte fuel cell stack 9 comprises a plurality of unit cells.
In the following, referring to FIG. 1, an operation of the conventional fuel cell power generating system will be described. As the fuel natural gas 1, the natural gas 45, the natural gas 46 and the natural gas 49 are supplied to the desulfurizer 2, the carbureter burner 35 and the reformer burner 53, respectively. The amount of the supplied natural gas 45 is set to a value appropriate to the cell current of the DC output power 22 and the temperature of the reformer 3(reformer temperature) by controlling the degree of opening of the flow control valve 37 based on a preset relationship among the cell current of the DC output power 22, the reformer temperature and the degree of opening of the flow control valve 37 (i.e. the amount of supplied natural gas 45).
The desulfurizer 2 removes sulfuric contents in an odorous material, such as mercaptan, in the natural gas 45, which cause deterioration of a reforming catalyst in the reformer 3 and an electrode catalyst of the anode 6 in the polymer electrolyte fuel cell stack 9, by hydrodesulfurization by the action of a cobalt-molybdenum-based catalyst, which is a desulfurizing catalyst, and a zinc-oxide adsorbent loading the desulfurizer 2. Specifically, the cobalt-molybdenum-based catalyst first causes reaction of sulfur and hydrogen to produce hydrogen sulfide, and then causes reaction of the resulting hydrogen sulfide and zinc oxide to produce zinc sulfide, thereby removing sulfuric contents. In order to supply hydrogen required to produce hydrogen sulfide, some of the reformed gas 26 rich in hydrogen, which is has been reduced in CO concentration to 1% or lower, is recycled and supplied to the desulfurizer 2 as the recycled reformed gas 50.
The amount of the recycled reformed gas 50 supplied is set to a value appropriate to the amount of the supplied natural gas 45 by controlling the degree of opening of the flow control valve 51 based on a preset relationship between the degree of opening of the flow control valve 37 (i.e. the amount of the supplied natural gas 45) and the degree of opening of the flow control valve 51 (i.e. the amount of the recycled reformed gas 50 supplied). The hydrodesulfurization and the reaction of producing zinc sulfide are both endothermic reactions. The heat required for the reactions is provided by supplying the heat produced by the water shift reaction in the CO shift converter 4, which is an exothermic reaction and will be described later, from the CO shift converter 4 to the desulfurizer 2.
The desulfurized natural gas 29 having the sulfuric content removed by the desulfurizer 2 is mixed with the steam 16 supplied from the carbureter 14, and the mixture gas 28 of the steam and the desulfurized natural gas is supplied to the reformer 3. The reformer 3 is filled with a nickel-based catalyst or a ruthenium-based catalyst serving as a reforming catalyst. The amount of the steam 16 mixed with the desulfurized natural gas 29 is set to such a value that a preset predetermined steam-carbon ratio (ratio of the steam to the carbon in the natural gas) is attained by controlling the degree of opening of the flow control valve 96 based on a preset relationship between the degree of opening of the flow control valve 37 (i.e. the amount of the supplied natural gas 45 for power generation) and the degree of opening of the flow control valve 96 (i.e. the amount of the supplied steam 16).
The carbureter 14 vaporizes the water 44 supplied from the water tank 90 by means of the pump 15. The heat required to vaporize the water 44 is provided by supplying the high-temperature combustion exhaust gas 24, described later, to the carbureter 14 and causing heat exchange between the water 44 and the combustion exhaust gas 24. The combustion exhaust gas 24 having heat exchanged with the water 44 is ejected as the exhaust gas 91. If the heat exchange between the water 44 and the combustion exhaust gas 24 does not provide enough heat to adequately vaporize the water 44 in the carbureter 14, combustion of the natural gas 46 supplied to the carbureter burner 35 via the flow control valve 47 and the air 31, which is some of the air 18 taken in by the air supply blower 13 and supplied to the carbureter burner 35 via the flow control valve 30, may provide additional heat to the carbureter 14.
The amount of the supplied natural gas 46 is set to such a value that a preset predetermined carbureter temperature is attained by controlling the degree of opening of the flow control valve 47 based on a preset relationship between the temperature of the carbureter 14 and the degree of opening of the flow control valve 47 (i.e. the amount of the supplied natural gas 46). Besides, the amount of the supplied air 31 is set to such a value that a preset predetermined air-fuel ratio (ratio of the air to the fuel) is attained by controlling the degree of opening of the flow control valve 30 based on a preset relationship between the degree of opening of the flow control valve 47 (i.e. the amount of the supplied natural gas 46) and the degree of opening of the flow control valve 30 (i.e. the amount of the supplied air 31).
To the water tank 90, the condensate 41 produced by the condenser 39 described later and the water 40 produced by the cell reaction in the polymer electrolyte fuel cell stack 9 are supplied. If they cannot adequately fill the water tank 90, the feed water pump 42 is activated, as required, to supply the feed water 43 to the water tank 90.
In the reformer 3, a steam reforming reaction of hydrocarbon contained in the natural gas is conducted by the action of the reforming catalyst loading the reformer 3, and thus, the reformed gas 27 rich in hydrogen is produced. The steam reforming reaction of methane, which is a primary component of the natural gas, is expressed by the following equation (1).
(Steam Reforming Reaction of Methane)CH4+H2O→CO+3H2  (1)
Steam reforming reactions of hydrocarbon including the steam reforming reaction of methane expressed by the equation (1) are endothermic reactions. Therefore, in order to efficiently produce hydrogen, heat required for the reaction has to be supplied from the outside of the reformer 3 and the temperature of the reformer 3 has to be maintained at 700 to 750 degrees C. Thus, the anode exhaust gas 19 containing about 20% of unreacted hydrogen, described later, is supplied from the polymer electrolyte fuel cell stack 9 to the reformer burner 53, and the air 34 which is some of the air 18 taken in by the air supply blower 13 is concurrently supplied to the reformer burner 53 to cause combustion thereof, thereby supplying heat required for the steam reforming reaction to the reformer 3. The amount of the supplied air 34 is set to such a value that a preset predetermined air-fuel ratio is attained by controlling the degree of opening of the flow control valve 12 based on a preset relationship between the degree of opening of the flow control valve 37 (i.e. the amount of the supplied natural gas 45) and the degree of opening of the flow control valve 12 (i.e. the amount of the supplied air 34).
If the combustion of the anode exhaust gas 19 in the reformer burner 53 does not provide enough heat for the steam reforming reaction of hydrocarbon in the reformer 3, combustion of the natural gas 49 supplied to the reformer burner 53 via the flow control valve 48 and the air 34, which is some of the air 18 taken in by the air supply blower 13 and supplied to the reformer burner 53 via the flow control valve 12, provides additional heat to the reformer 3.
The amount of the supplied natural gas 49 is set to a value appropriate to a preset predetermined temperature of the reformer 3 by controlling the degree of opening of the flow control valve 48 based on a preset relationship between the temperature of the reformer 3 and the degree of opening of the flow control valve 48 (i.e. the amount of the supplied natural gas 49). Besides, the amount of the supplied air 34 is set to such a value that a preset predetermined air-fuel ratio is attained by controlling the degree of opening of the flow control valve 12 based on a preset relationship between the degree of opening of the flow control valve 37 (i.e. the amount of the supplied natural gas 45) and the degree of opening of the flow control valve 12 (i.e. the amount of the supplied air 34).
The reformed gas 27 rich in hydrogen, which is an exhaust gas from the reformer 3, contains carbon monoxide, which causes deterioration of the electrode catalyst at the anode 6 of the polymer electrolyte fuel cell stack 9. Therefore, the reformed gas 27 rich in hydrogen is supplied to the CO shift converter 4 loaded with a CO shift converter catalyst, such as copper-zinc-based catalyst, thereby reducing the concentration of CO in the reformed gas 27 rich in hydrogen to 1% or lower by water shift reaction due to the CO shift converter catalyst, the water shift reaction being expressed by the following equation (2).
(Water Shift Reaction)CO+H2O→CO2+H2  (2)
The water shift reaction is an exothermic reaction. The heat generated is supplied to the desulfurizer 2 and used for the hydrodesulfurization and the reaction of producing zinc sulfide in the desulfurizer 2 described above, which are endothermic reactions.
Part of the reformed gas 26, which is an exhaust gas from the CO shift converter 4, is supplied to the desulfurizer 2 as the recycled reformed gas 50 as described above, and the remainder thereof is supplied, as the reformed gas 52, to the CO selective oxidizer 5 loaded with a precious metal catalyst, such as platinum-based catalyst or ruthenium-based catalyst, serving as a CO selective oxidizing catalyst. This is intended to reduce the CO concentration of the reformed gas 52 to about 10 ppm, because a reformed gas having a CO concentration of 100 ppm or higher supplied to the anode 6 causes deterioration of the electrode catalyst. In addition, the air 33 which is some of the air 18 taken in by the air supply blower 13 is supplied to the CO selective oxidizer 5. The CO selective oxidizer 5 causes carbon monoxide contained in the reformed gas 52 to react with oxygen in the air 33 to convert carbon monoxide into carbon dioxide through a CO selective oxidizing reaction expressed by the following equation (3), which is an exothermic reaction, thereby reducing the CO concentration of the reformed gas 52 to about 10 ppm.
(CO Selective Oxidizing Reaction of Carbon Monoxide)CO+1/2O2→CO2  (3)
The amount of the supplied air 33 is set to a value appropriate to the amount of the supplied natural gas 45 by controlling the degree of opening of the flow control valve 11 based on a preset relationship between the degree of opening of the flow control valve 37 (i.e. the amount of the supplied natural gas 45) and the degree of opening of the flow control valve 11 (i.e. the amount of the supplied air 33).
Unreacted steam contained in the reformed gas 25 having the CO concentration reduced to about 10 ppm, which is an exhaust gas from the CO selective oxidizer 5, is cooled to a temperature of 100 degrees C. or lower and collected as the condensate 41 in the condenser 39. The condensate 41 is supplied to the water tank 90 and reused as the water 44 to be supplied to the carbureter 14. The reformed gas 38, which results from condensation of unreacted steam in the condenser 39, is supplied to the anode 6.
On the other hand, the air 32 which is some of the air 18 taken in by the air supply blower 13 is supplied to the cathode 8 of the polymer electrolyte fuel cell stack 9. Typically, the power generating temperature of the polymer electrolyte fuel cell stack 9 is 60 degrees C. to 80 degrees C. The power generating temperature is maintained by heat generated by the cell reaction. The amount of the supplied air 32 is set to a value appropriate to the cell current of the DC output power 22 by controlling the degree of opening of the flow control valve 10 based on a preset relationship between the cell current of the DC output power 22 and the degree of opening of the flow control valve 10 (i.e. the amount of the supplied air 32).
At the anode 6, by the action of a platinum-based electrode catalyst, about 80% of hydrogen contained in the reformed gas 38 is changed into hydrogen ions and electrons through the anode reaction expressed by the following equation (4).
(Anode Reaction)H2→2H++2e−  (4)
The hydrogen ions produced at the anode 6 move in the solid polymer electrolyte 7 composed of a fluorine-based polymer having a sulfonic group, such as Nafion, and reach the cathode 8. On the other hand, the electrons produced at the anode 6 move through an external circuit (not shown) and reach the cathode 8. In the process of the electrons moving through the external circuit, electric energy can be extracted as the DC output power 22.
At the cathode 8, by the action of the platinum-based electrode catalyst, the hydrogen ions having moved from the anode 6 to the cathode 8 through the solid polymer electrolyte 7, the electrons having moved from the anode 6 to the cathode 8 through the external circuit, and the oxygen in the air 32 supplied to the cathode 8 react with each other to form water, the reaction being referred to as a cathode reaction expressed by the following equation (5).
(Cathode Reaction)2H++1/2O2+2e−→H2O  (5)
Bringing together the equations (4) and (5), the cell reaction in the polymer electrolyte fuel cell stack 9 can be expressed as a reverse reaction of the electrolysis of water, in which hydrogen and oxygen react with each other to form water, as expressed by the following equation (6).
(Cell Reaction)H2+1/2O2→H2O  (6)
The power adjusting device 20 performs voltage conversion and DC/AC conversion on the DC output power 22 generated by the polymer electrolyte fuel cell stack 9 to make it suitable for the load 21 and then supplies the resulting AC output power 23 to the load 21. While FIG. 1 shows an example in which the power adjusting device 20 performs DC/AC conversion, the power adjusting device 20 may perform only voltage conversion and the sending end DC output power may be supplied to the load 21.
The reformed gas 38 is ejected as the anode exhaust gas 19 of the polymer electrolyte fuel cell stack 9 after about 80% of hydrogen therein is consumed at the anode 6 by the anode reaction expressed by the equation (4). On the other hand, the air 32 is ejected as the cathode exhaust gas 17 of the polymer electrolyte fuel cell stack 9 after some of oxygen therein is consumed at the cathode 8 by the cathode reaction expressed by the equation (5).
The water 40 produced in the polymer electrolyte fuel cell stack 9 by the cell reaction expressed by the equation (6) is supplied to the water tank 90, as with the condensate 41, and reused as the water 44 supplied to the carbureter 14. Since about 20% of hydrogen in the reformed gas 38 remains in the anode exhaust gas 19 without being reacted, the anode exhaust gas 19 is used as fuel for the reformer burner 53 as described above.
The conventional fuel cell power generating system shown in FIG. 1 has problems described below. In order to cause the steam reforming reaction of hydrocarbon contained in the natural gas 45 in the anode exhaust gas 19 from the polymer electrolyte fuel cell stack 9 in the reformer 3, not only the anode exhaust gas 19 from the polymer electrolyte fuel cell stack 9 but also the natural gas 49 for the reformer burner 53 have to be supplied to the reformer burner 53 for combustion. Furthermore, since the power generating temperature of the polymer electrolyte fuel cell stack 9 is low, specifically 60 degrees C. to 80 degrees C., no steam can be produced in the process of cooling the cell stack, unlike the case of using a phosphoric acid fuel cell stack having a power generating temperature of 190 degrees C. Therefore, the carbureter 14 has to be provided for heat exchange with the combustion exhaust gas 24, and combustion of the natural gas 46 supplied to the carbureter burner 35 has to be caused to externally supply to the carbureter 14 the heat required to vaporize the water 44, thereby producing the steam 16, which is required for steam reforming reaction of hydrocarbon in the reformer 3. Thus, the conventional fuel cell power generating system has a low sending end efficiency, specifically, lower than 40% (Low Heat Value (LHV) reference, the same in the following). In addition, since the sending end efficiency is low, the AC output power at the sending end is also low.